Portable communication devices, such as cellular telephones, portable computers, personal digital assistants (PDAs), Global Navigation Satellite System (GNSS) (e.g., including global positioning system (GPS)) receivers, and the like, are configured to communicate over wireless networks. Such portable communication devices may enable communication over multiple networks, and therefore include transmitters, receivers and corresponding filters in multiplexer devices, connecting the receivers and transmitters to a common antenna, for sending and receiving signals (e.g., radio frequency (RF) signals) over the various networks.
A multiplexer device interfaces between the antenna and each of the networks to enable transmitting signals on different transmit (uplink) frequencies and receiving signals on different receive (downlink) frequencies. The filters associated with the multiplexer device include band pass filters and band stop filters (or notch filters). Generally, the band pass filters provide passbands for passing various transmitted and received signals through relatively narrow frequency bands (blocking all signals with frequencies outside the passbands), while the notch filters provide stopbands for blocking various transmitted and received signals in relatively narrow frequency bands (passing all signals with frequencies outside the stopbands). The band pass filters and notch filters may be used in a complementary fashion, such that a band pass filter associated with a first network has a passband that corresponds to (e.g., matches) a stopband of a notch filter associated with a second network. In this manner, the likelihood of the signals passing through the band pass filter interfering with the signals of the second network is greatly reduced.
FIG. 1A is a block diagram showing an example of a conventional multiplexer device providing band pass and notch filters for different networks, as well as an inductor-capacitor (LC) diplexer consisting of high pass and low pass filters. FIG. 1B is a graph showing a typical notch filter transmission response (insertion loss) of the conventional multiplexer having only acoustic notch filters.
In FIG. 1A, multiplexer device 100 is arranged in a cascading topology. That is, a common antenna port 105 is connected in parallel between first band pass filter 111 and corresponding first notch filter (or band stop filter) 112 in a first notch filter path 110. The antenna port 105 is connected to an antenna 108 for receiving and/or transmitting signals, such as RF signals, corresponding to various types of networks. The output of the first notch filter 112 is connected in series between second band pass filter 121 and corresponding second notch filter 122 in a second notch filter path 120. This series connection adds additional losses to both notch filter 122 and band pass filter 121. Each of the first band pass filter 111, the first notch filter 112, the second band pass filter 121 and the second notch filter 122 is an acoustic filter, such as a ladder-type acoustic band pass filter, formed of multiple series and shunt connected acoustic resonators.
Since a typical acoustic notch filter, such as first and second notch filters 112 and 122, does not provide sufficient rejection in corresponding non-acoustic areas, additional filtering is required. For example, FIG. 1B is a graph showing insertion loss (in decibels (dB)) versus frequency (in gigahertz (GHz)) of a WiFi acoustic notch filter. As shown by trace 140 in FIG. 1B, a representative acoustic notch filter provides a stopband 144 between about 2.4 GHz and about 2.5 GHz. Notably, the acoustic notch filter provides practically no attenuation (apart from the stopband) over the entire depicted frequency range of 0.5 GHz to 3.0 GHz, in which most wireless applications operate.
Thus, in order to make the wireless system more efficient (for example, using different front-end modules for low and high cell bands), the acoustic notch filter can be separated into two or more narrower frequency ranges. In FIG. 1A, for example, LC diplexer 130 is connected in series to the second notch filter 122. The LC diplexer 130 separates the second notch filter path 120 into a low pass filter (LPF) 131 and a high pass filter (HPF) 132. Thus, the LPF 131 filters out the higher frequencies, while the HPF 132 filters out the lower frequencies, as well as frequency ranges corresponding to the stopbands of the first and second notch filters 112 and 122. As mentioned above, in the conventional multiplexer device 100, each of the LPF 131 and the HPF 132 is an LC filter. Generally, an LC filter is formed of inductor(s) and capacitor(s), although some resistance incidental to circuit design and implementation may also appear in an LC filter. In the context of the multiplexer devices discussed herein, including multiplexer devices 100, 200 and 300, an LC filter includes inductor(s) and capacitor(s) as elements in the filter architecture, but no acoustic resonators.
The topology of the conventional multiplexer device 100 has a number of drawbacks. For example, the insertion loss of the second band pass filter 121 is higher than that of the first band pass filter 111 due to the cascading topology. The insertion loss of second notch filter path 120 is even more impacted, as compared to the insertion loss of the first notch filter path 110, by the additional LC diplexer 130 connected in series. The LC diplexer 130 architecture also requires additional area for the second notch filter path 120.